[PATCH v2 26/28] serial: dm: Add support for puts

[Sean Anderson]

Some serial drivers can be vastly more efficient when printing multiple
characters at once. Non-DM serial has had a puts option for these sorts
of drivers; implement it for DM serial as well.

Because we have to add carriage returns, we can't just pass the whole
string directly to the serial driver. Instead, we print up to the
newline, then print a carriage return, and then continue on. This is
less efficient, but it is better than printing each character
individually. It also avoids having to allocate memory just to add a few
characters.

Drivers may perform short writes (such as filling a FIFO) and return the
number of characters written in len. We loop over them in the same way
that _serial_putc loops over putc.

This results in around 148 bytes of bloat for all boards with DM_SERIAL
enabled:

vexpress_aemv8a_juno: all +148 rodata +8 text +140
   u-boot: add: 2/0, grow: 0/-2 bytes: 232/-92 (140)
	 function                                   old     new   delta
	 _serial_puts                                 -     200    +200
	 strchrnul                                    -      32     +32
	 serial_puts                                 68      24     -44
	 serial_stub_puts                            56       8     -48

Signed-off-by: Sean Anderson <sean.anderson at seco.com>
---

Changes in v2:
- New

 drivers/serial/serial-uclass.c | 27 +++++++++++++++++++++++++--
 include/serial.h               | 18 ++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index 362cedd955..352ad986f7 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -199,8 +199,31 @@ static void _serial_putc(struct udevice *dev, char ch)
 
 static void _serial_puts(struct udevice *dev, const char *str)
 {
-	while (*str)
-		_serial_putc(dev, *str++);
+	struct dm_serial_ops *ops = serial_get_ops(dev);
+
+	if (!ops->puts) {
+		while (*str)
+			_serial_putc(dev, *str++);
+	}
+
+	do {
+		const char *newline = strchrnul(str, '\n');
+		size_t len = newline - str + !!*newline;
+
+		do {
+			int err;
+			size_t written = len;
+
+			err = ops->puts(dev, str, &written);
+			if (err && err != -EAGAIN)
+				return;
+			str += written;
+			len -= written;
+		} while (len);
+
+		if (*newline)
+			_serial_putc(dev, '\r');
+	} while (*str);
 }
 
 static int __serial_getc(struct udevice *dev)
diff --git a/include/serial.h b/include/serial.h
index 2681d26c82..ea96d904d8 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -195,6 +195,24 @@ struct dm_serial_ops {
 	 * @return 0 if OK, -ve on error
 	 */
 	int (*putc)(struct udevice *dev, const char ch);
+	/**
+	 * puts() - Write a string
+	 *
+	 * This writes a string. This function should be implemented only if
+	 * writing multiple characters at once is more performant than just
+	 * calling putc() in a loop.
+	 *
+	 * If the whole string cannot be written at once, then @len should be
+	 * set to the number of characters written, and this function should
+	 * return -EAGAIN.
+	 *
+	 * @dev: Device pointer
+	 * @s: The string to write
+	 * @len: The length of the string to write. This should be set to the
+	 *       number of characters actually written on return.
+	 * @return 0 if OK, -ve on error
+	 */
+	int (*puts)(struct udevice *dev, const char *s, size_t *len);
 	/**
 	 * pending() - Check if input/output characters are waiting
 	 *
-- 
2.25.1